The present invention pertains to systems for measuring and indicating the depth of material in a container, such as a bin containing fertilizer.
There are many instances in which it is desirable or necessary to provide a quick measurement of the amount of material stored in a container. In a typical industrial or agricultural setting, a bin or other container is provided for temporary storage of some material, such as fertilizer, grain, feed, cement, carbon black, etc. It is typical for quantities of material to be added to or withdrawn from the container from time to time, and the need arises for knowing the amount of material in the container at any given moment.
It is not realistically possible to provide actual measurement of quantities of material added to and withdrawn from the container, in order to maintain a tally of its actual contents. Such schemes are generally not practical because of the length of time and degree of compleixty involved in making actual measurements of the material. Rather, it is generally preferable to allow for rapid adding or subtracting of material from the container, such as by conveyor, dump truck, auger, or the like, without the necessity of actual measurement of the material so added or removed. This in turn implies a need for an indirect means of measurement, and preferably one which is quickly and accurately made.
Since the total volume of a given container is known, having once been measured or calculated, the simplest method of measuring material in such container involves measurement of the depth of the material at a given time. Because visual inspection is generally inconvenient or impractical, various systems have been proposed in the prior art for measurement of the depth of the material by means of a sensor or sensors placed within the container, and some type of readout means located externally of the bin, such as an office which contains readouts for numerous containers in a given installation.
Sensors for measuring depth of granular material in containers have been proposed which operate electrically, or by fluid pressure. In either case, an elongate sensor or a plurality of sensors may be placed vertically in the container so as to be progressively immersed in the material as it is added to the container.
Prior art fluid type systems have been proposed, which involve the use of an elongate sensor mounted vertically in the bin. The sensor has a movable or flexible portion which is intended to be compressed by the accumulating material in the bin so as to displace a quantity of fluid from within the sensor. The fluid bellow or the like is then provided to measure the displaced fluid.
Regardless of whether operated by mechanical or electrical means, prior art systems have suffered from certain problems and inaccuracy due to the amount of force required to actuate or compress the sensor. This is because most dry products develop horizontal forces which are very low compared to the weight per cubic foot of material. This is especially true of very light materials such as carbon black or sawdust, in which the very low horizontal forces approach the limits of the sensitivity of prior art sensors.
Typical prior art fluid systems use a flexible member or diaphragm defining an air passage, positioned vertically in the container. A fluid communication line from the top of the sensor connects to a bellows, piston, or other displacement indicating device, to which is attached an indicating pointer or other readout means. An example of such a system is found in U.S. Pat. No. 3,401,562, issued to W. A. Reaney. As material in the container causes compression of the sensor, the bellows or piston is caused to move in response. Such systems are subject to a major problem of temperature sensitivity, in addition to the fact of product bridging and the weakness of the horizontal forces developed in the product as discussed above, upon which such systems must rely for compression of their sensors.
In this type of prior art system, the entire fluid system including the sensor, interconnecting line, and the bellows or other readout device must be sealed from the atmosphere. Unfortunately, this renders the system highly susceptible to erroneous readings caused by temperature changes. When the temperature increases, the air in the system expands, giving erroneous readings, and vice versa when the temperature drops. Of course, use of a liquid instead of a gas as the working fluid would help in this respect, but it is generally not feasible to do so, because the density of the liquid would build up a significant pressure head in the elongate vertical sensor, requiring excessive and unrealistic displacement forces to be supplied by the material.
In order to overcome temperature problems, systems have been proposed in the prior art which include elaborate temperature compensating bellows, as shown in U.S. Pat. No. 3,290,938 issued to R. R. Miller for example. Unfortunately, this proposed solution leads to greater complexity and increased costs, and potentially increases the vulnerability of the system to leaks. In addition, it tends to clog.
Because the prior art fluid systems depend upon a completely sealed fluid system, the presence of even a minute leak will seriously affect long term accuracy. Although it is possible to build a system relatively free of gross leaks, the extent of the sensor, and the other tubes and devices involved in the measurement system makes it extremely difficult to guard against long term, slow leaks which will degrade accuracy over a period of weeks or months. As a generality, recalibration in this type of prior art system is not feasible, short of completely emptying or completely filling the container. Also, pressure does not activate these systems.
The present invention solves these and other problems existing in the art by providing an improved depth measurement system which takes advantage of the inherent simplicity and economies of a fluid system, but which works upon a different principle so as to avoid the problems heretofore existing in the art. Clogging of the components of my novel system is virtually non-existent.